System and method for enhanced accuracy in cable diagnostics of cable length

ABSTRACT

A system and method for enhanced accuracy in cable diagnostics of cable length. Conventional cable diagnostics such as time domain reflectometry can be used to determine cable length. This conventional technique can have accuracy limitations in certain situation such as with perfectly terminated cable. A cable length can also be determined through the use of link delay measurements that are based on clock synchronization between nodes in a network. Notwithstanding the accuracy issues of these link delay measurements, overall accuracy can be increased through the combination of the two cable length delay measurements into a final estimate.

This application is a continuation of application Ser. No. 11/844,497,filed Aug. 24, 2007 now U.S. Pat. No. 7,830,152 issued Nov. 9, 2010,which is incorporated by reference herein, in its entirety, for allpurposes.

BACKGROUND

1. Field of the Invention

The present invention relates generally to Ethernet networks and, moreparticularly, to a system and method for enhanced accuracy in cablediagnostics of cable length.

2. Introduction

Cable diagnostic play a key role in the management of various networks.Various forms of cable diagnostics can be used to detect and identifycable faults, cable length, cable topologies, etc. An identification ofcable length is particularly useful in determining the potentialperformance of a link over that cable, which enables diagnostics and/orenables other applications.

To illustrate such an impact, consider an example application such aspower over Ethernet (PoE). In a PoE application such as that describedin the IEEE 802.3af and 802.3at specifications, a power sourcingequipment (PSE) delivers power to a powered device (PD) over Ethernetcabling. Various types of PDs exist, including voice over IP (VoIP)phones, wireless LAN access points, Bluetooth access points, networkcameras, computing devices, etc. In an enterprise network, various PDscan be deployed on a permanent or non-permanent basis at variouslocations (e.g., conference rooms) situated throughout the enterprisefacility.

In powering these various PDs, a key piece of information is the lengthof the cable between the PSE and the PD. In one example, this lengthinformation can be used to calculate or otherwise determine a resistanceof the cable link. The resistance of the cable link in turn can be usedto identify a voltage drop and/or power loss that can be attributed tothe cable link. For a given PD deployment, the voltage drop and/or powerloss can play a key role in the determination and allocation of anaccurate power budget attributable to that port. As the resistance ofthe cable is proportional to the length of the cable, an accuratedetermination of the cable length is critical to any calculations orassessments that are based on the resistance of the cable link.

As would be appreciated, the determined length of the cable can beuseful in various diagnostic capacities either alone or in combinationwith the particular needs of a given application. There are manyapplications beyond PoE that can use the cable length information. Whatis needed therefore is a mechanism that enables enhanced accuracy in thecable diagnostics of cable length.

SUMMARY

A system and/or method for enhanced accuracy in cable diagnostics ofcable length, substantially as shown in and/or described in connectionwith at least one of the figures, as set forth more completely in theclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to describe the manner in which the above-recited and otheradvantages and features of the invention can be obtained, a moreparticular description of the invention briefly described above will berendered by reference to specific embodiments thereof which areillustrated in the appended drawings. Understanding that these drawingsdepict only typical embodiments of the invention and are not thereforeto be considered limiting of its scope, the invention will be describedand explained with additional specificity and detail through the use ofthe accompanying drawings in which:

FIG. 1 illustrates an example of an Ethernet network that enablesconnectivity of client and server devices.

FIG. 2 illustrates an embodiment of a device that supports networksynchronization.

FIG. 3 illustrates a flowchart of a process of generating a cable lengthestimate.

DETAILED DESCRIPTION

Various embodiments of the invention are discussed in detail below.While specific implementations are discussed, it should be understoodthat this is done for illustration purposes only. A person skilled inthe relevant art will recognize that other components and configurationsmay be used without parting from the spirit and scope of the invention.

In a conventional cable diagnostic process, the length of a cablebetween two network devices can be determined through an identificationof discontinuities in the cable. For example, time domain reflectometry(TDR) performed by a physical layer device (PHY) can be used to detectthose discontinuities through the measurement of reflections of a signalthat is injected into the cable. Here, the time interval betweenlaunching the injected signal and receiving a reflection of the injectedsignal is linearly proportional to the cable length. The cable lengthcan then be computed by multiplying the media propagation speed with theidentified time interval. The calculated result is then divided by twoto account for the round-trip delay of the measured signal.

In some situations, the cable length that is calculated using the TDRmeasurements can be inaccurate. This can result, for example, where thecable is perfectly terminated at the other end of the link. As noted,these inaccuracies can significantly impact applications such as PoEthat rely on cable length information for configuration purposes.

Another mechanism by which cable length can be determined is throughlink delay measurements. These link delay measurements can be based onmessages that are passed between network devices that have anestablished common time reference. This common time reference enablesnetwork devices to use timestamps that are derived, in effect, from thesame source clock. By including a timestamp in a message, the receivingnetwork device is able to identify a time of transmission of themessage. The difference between the time of transmission and the time ofreception can provides an estimate of the time of flight of the message.This time of flight can be used to estimate a distance in the link. Aswould be appreciated, calculations based on a round trip time of flightcan be used to generate a better estimate of the length of the cablelink.

In one embodiment, the common time reference between network devices canbe established as part of a connection reservation protocol such asaudio/video (AV) bridging, which can be applied to AV streaming acrossthe network. In general, AV bridging such as that described in IEEE802.1 has been developed to reserve a connection with a certain qualityof service (QoS). In this process, a bandwidth reservation protocol anda time synchronization protocol would be implemented to reserve aconnection with guaranteed levels of bandwidth and latency.

Latency, in particular, is a significant issue and would require theperiodic exchange of timing information that would allow both ends ofthe link to synchronize their time-of-day clock precisely. In oneembodiment, different granularities can be used to meet differenttraffic classes. For example, 125 μs periods (used in most currentisochronous transports) can be used for low latency streams, while 1 msperiods can be used for moderate latency streams.

During link establishment, AV devices would exchange capabilityinformation. If the devices have the same network synchronizationcapability, the devices would then exchange configuration and clocksynchronization information. Bridges between the devices would similarlybe involved in the exchange of configuration and synchronizationinformation. If all links in the connection between the devices cansupport network synchronization, then the connection having a certainQoS can be reserved. In contrast, if one of the links in the connectionbetween the devices cannot support network synchronization, then theconnection having a certain QoS cannot be reserved.

FIG. 1 illustrates an example of a connection between AV devices 110 and150. In this example, a connection between AV devices 110, 150 issupported by AV switches 120, 130, 140. If each of AV devices 110, 150and AV switches 120, 130, 140 support network synchronization, then aconnection having a certain QoS can be reserved between AV devices 110,150. In contrast, if any of AV devices 110, 150 and AV switches 120,130, 140 do not support network synchronization, then a connectionhaving a certain QoS cannot be reserved between AV devices 110, 150.

FIG. 2 illustrates an embodiment of a device that enables networksynchronization. As illustrated, the device would include timestamp shim230, which is designed to generate timestamps based on a common timereference that has been established between devices such as thatexemplified in FIG. 1. Ethernet signals that include such timestamps aresent to a receiving network device through media access controller (MAC)220 and Ethernet PHY 210. In various embodiments, one or more MACclients 240 can be used that can be coupled to an I/O controller,graphics subsystem, etc. When operating as an end device, MAC client 240can include a traffic classifier and scheduler.

In general, AV switches can be designed to include multiple MACs andPHYs, while also including admission controller, framefiltering/routing, and time synchronization services. Additional detailsof such services are exemplified by IEEE 802.1AS, which provides a timesynchronization protocol; IEEE 802.1Qat, which provides a streamreservation protocol; and IEEE 802.1Qav, which provides for guaranteedlatency and bandwidth for established streams.

One of the limitations of link delay measurements is the timesynchronization granularity. For typical Ethernet connection speeds, thetime synchronization granularity is 40 ns. At this time synchronizationgranularity, the cable length uncertainty for the media propagationspeed of Ethernet cable is on the order of ±8 m. As a typical Ethernetcable length is less than 100 m, this cable length uncertainty canrepresent a significant accuracy limitation for the link delaymeasurement technique. Another limitation of link delay measurements isthat the link needs to be active for network synchronization to occur.

In the present invention, it is recognized that neither the link delaymeasurement technique nor the TDR technique can be used on their own foraccurate cable diagnostics. It is therefore a feature of the presentinvention that the two techniques can be used in combination to producea more accurate and reliable cable length estimate. In typical deviceimplementations, both the link delay measurement and TDR techniques areperformed by the same subsystem of the device. Accordingly,consideration of the results of both techniques can be performed by aprocess such as that illustrated in the flowchart of FIG. 3.

As illustrated, the process begins at step 302 where a first lengthestimate is determined based on detection of a discontinuity. As notedabove, this length estimate can be generated using TDR or othermeasurements that examine the reflection of a signal that is insertedinto the cable.

Next, at step 304, network synchronization is established betweennetwork devices. In one embodiment, the network synchronization isenabled through AV bridging technology such as that described above. Ingeneral, any network synchronization protocol that establishes a commontime reference between devices can be used. Here, it should be notedthat the network synchronization is established between at least the twodevices that are on either end of the cable being measured. While thenetwork synchronization can be established between multiple devices thatcreate a link over multiple hops (e.g., peer devices along withintermediate bridge/switch devices such as that illustrated in FIG. 1),the network synchronization would enable link delay measurements over asingle hop.

After network synchronization is established at step 304, a secondlength estimate is determined based on a link delay measurement at step306. Regardless of the relative timing in generating the lengthestimates, the two length estimates can be used to generate a moreaccurate and/or reliable length estimate. In one embodiment, an analysisof the first and second length estimates is performed at step 308. Here,the analysis can consider whether the first and second length estimatesare wildly divergent. For example, the first length estimate can beseriously deficient if the cable is perfectly terminated. In anotherexample, the second length estimate can be seriously deficient if anetwork synchronization error has occurred. In the case of wildlydivergent results (e.g., results that differ by an amount far greaterthan the length measurement uncertainty), the system can be designed toeliminate one of the results and report the other length estimate as thefinal length estimate at step 310.

Where the two length estimates are not wildly divergent, the system canbe designed to combine the two length estimates into the final lengthestimate. For example, at step 310, the system can choose to simplyaverage the two length estimates. In another example, the system canchoose to weight the two length estimates should there be indicationsthat one of the length estimates is based on questionable readings.

In various implementations, one of the length estimates can beeliminated or weighted more than the other length estimate based onvarious criteria such as application used, accuracy desired, repeatedreadings on each type of application, clock considerations, etc.

Regardless of the method by which the length estimates are combined, thecombination will produce a more accurate result across a variety ofoperating conditions. Where the length result is used in some form ofsystem configuration (e.g., PoE), improvements in system operation wouldalso result.

It should be noted that the principles of the present invention canapply to various PHY speeds such as 10BASE-T, 100BASE-TX, 1000BASE-T,10GBASE-T, etc, apply to non-standard PHY speeds such as 2.5G, 5G, etc.,or apply to various cable types such as Cat 3, 5, 5e, 6 and 7. Also, theprinciples of the present invention would not be dependent on clockresolutions within the network synchronization scheme.

These and other aspects of the present invention will become apparent tothose skilled in the art by a review of the preceding detaileddescription. Although a number of salient features of the presentinvention have been described above, the invention is capable of otherembodiments and of being practiced and carried out in various ways thatwould be apparent to one of ordinary skill in the art after reading thedisclosed invention, therefore the above description should not beconsidered to be exclusive of these other embodiments. Also, it is to beunderstood that the phraseology and terminology employed herein are forthe purposes of description and should not be regarded as limiting.

1. A cable diagnostic method in a first network node, comprising:receiving, in said first network node from a second network node, amessage that includes timing information based on a common timereference established between said first network node and said secondnetwork node; determining a first estimate of a length of a cablecoupling said first network node and said second network node using saidtiming information; and determining a final estimate of said length ofsaid cable using said first estimate and a second estimate of saidlength of said cable, said second estimate being generated based on adetection of a reflection at a discontinuity in said cable.
 2. Themethod of claim 1, wherein said common time reference is establishedusing a connection reservation protocol.
 3. The method of claim 1,wherein said determination of said first estimate is based on adifference of a packet transmission time and a packet reception time ina one-way trip.
 4. The method of claim 1, wherein said final estimate isselected from one of said first estimate and said second estimate. 5.The method of claim 1, wherein said final estimate is an average of saidfirst estimate and said second estimate.
 6. The method of claim 1,wherein said final estimate is a weighted average of said first estimateand said second estimate.
 7. The method of claim 1, wherein said finalestimate is determined based on consideration of an application oraccuracy.
 8. A cable diagnostic method in a first network node of anetwork, comprising: receiving, in said first network node, a reflectionfrom a discontinuity in a cable coupling said first network node to asecond network node; determining a first estimate of a length of saidcable based on a round trip time determined from a time of receipt ofsaid reflection; and determining a final estimate of said length of saidcable using said first estimate and a second estimate of said length ofsaid cable, said second estimate being generated based on a messagereceived from said second network node that includes timing informationbased on a common time reference established between said first networknode and said second network node.
 9. The network device of claim 8,wherein said common time reference is established using a connectionreservation protocol.
 10. The method of claim 8, wherein saiddetermination of said second estimate is based on a difference of apacket transmission time and a packet reception time in a one-way trip.11. The method of claim 8, wherein said final estimate is selected fromone of said first estimate and said second estimate.
 12. The method ofclaim 8, wherein said final estimate is an average of said firstestimate and said second estimate.
 13. The method of claim 8, whereinsaid final estimate is a weighted average of said first estimate andsaid second estimate.
 14. The method of claim 8, wherein said finalestimate is determined based on consideration of an application oraccuracy.
 15. A cable diagnostic method in a first node of a network,comprising: receiving a first estimate of a length of a cable that isdetermined using a reflection of a signal that is injected into saidcable; receiving a second estimate of a length of said cable that isdetermined in said first node using a link delay measurement that isbased on a receipt of a message from a second node in said networkhaving a common time reference with said first node; and determining afinal estimate of said length of said cable upon consideration of saidfirst estimate and said second estimate.
 16. The method of claim 15,wherein said common time reference is established using a connectionreservation protocol.
 17. The method of claim 15, wherein said linkdelay measurement is based on a difference of a packet transmission timeand a packet reception time.
 18. The method of claim 15, wherein saidfinal estimate is selected from one of said first estimate and saidsecond estimate.
 19. The method of claim 15, wherein said final estimateis an average of said first estimate and said second estimate.
 20. Themethod of claim 15, wherein said final estimate is a weighted average ofsaid first estimate and said second estimate.